1. Field of the Invention
The present invention relates to a method for fining powders and an apparatus employing the same, which can particularly reduce sizes of fragile powders from D50<20 μm to D50<2.0 μm with advantages of dry, low-temperature, saving-power, less consumed material and high efficiency.
2. Related Prior Arts
Conventionally, the powders are usually fined in a static state by applying pressure thereon, i.e., grinding or milling.
However, sizes of the fined powders are limited by applying the conventional technologies. The milling tool has to be replaced with a new one when the surface thereof is too rough to mill the powders. The applied kinetic energy will be lost as friction occurs between the milling tool and the powders and is transformed into heat. For example, the ball mill can mill cement powders to a specific area about 3,500 and D50<20 μm, but more milling will be not economic or result in aggregation. In principle, each of the raw powders has to be milled as 1,000 fine powders if a specific area 11,000 and a diameter D50<2.0 μm are desired, and it is impossible for current milling tools to achieve in a short period.
The current fining technologies can be classified into dry processes and wet (attriting) processes. For cement capable of hydration with water, dry processes are desired, for example, by ball mill or roller mill. The pulverized powders are delivered to a screening or sieving machine to collect powders with desired diameter. However, it's difficult to acquire cement powders with D50<20 μm by the conventional processes, and therefore high-pressure mills are developed. JP No. 8243427, JP No. 1284342, JP No. 8164345 and CN No. 1593771 provided similar apparatuses, in which several rollers are used to press and grind the powders and intervals between the rollers are adjustable. Another dry process is developed in which the powders are carried with the high-pressure gas flow and collide with each other. For example, CN No. 1,483,516 and JP No. 2002-079133 disclosed a jet flow or a special nozzle to fast deliver the powders and achieve collisions therebetween. However, collisions between the powders are not efficient as the powders are too small and will exit from the chamber very soon with the high-pressure air flow. This process consumes a lot of electricity and has unsatisfying yield. U.S. Pat. No. 5,839,670 combined the rollers and a jet flow to promote the fining effect. JP No. 2005-205266, JP No. 2005-177704, JP No. 2002-346411 and U.S. Pat. No. 5,354,002 further provided apparatuses having multiple cones or recycling ducts which achieve a sorting effect according to specific distribution of air flows. Such structures perform both pulverization and sorting effects. For the above attriting mill, proper media (grinding aid) are needed to avoid aggregation and cool the material, which absolutely increases cost and causes the problem of recycling the media.
In general, the powders can be fined either by collisions therebetween or colliding with a fix structure, and the fining effect varies with energy thereof. Further, the fined powders can be sorted according to deviation of the gas flows. To overcome the problems of the conventional technologies, the present invention provides a novel method and apparatus efficiently fine the powders with lower cost.